The invention relates generally to flexible and stretchable electronics. More specifically, the invention relates to an integrated device comprising a substrate with an embedded rigid electronic device, where the substrate has a stiffness gradient around the embedded device to allow flexible and stretchable movement of the integrated device.
Flexible and stretchable electronics have emerged as a new technology for realizing smart sensors and actuators for applications ranging from medicine to personal electronic devices. Such systems have been evolving at a rapid rate with the promise of integration into areas such as the human body. However, integrating rigid electronics with a flexible substrate (i.e. the human body) poses problems resulting from the mismatch in compliance between the two materials.
Stretchable electronics have been pursued through a wide variety of techniques including organic electronic materials such as conductive polymers, nanowires, microfluidic circuits, and thin inorganic materials patterned on soft polymers. An elusive goal of these approaches is to simultaneously achieve the performance and reliability of established foundry electronic devices in a stretchable platform. However, the goal is unmet as these designs suffer from relatively poor transistor density and performance in addition to uncertain reliability.
Inorganic materials such as silicon processors have been used in electronic devices for decades and embedding these materials in stretchable and flexible structures would provide integrated functionality and reliability. However, delamination of the rigid processor from the soft substrate has inhibited the impact of this approach. To overcome this problem, one method attempts to use sub-micron layers of inorganic materials within an electronic device, which allows the stiff materials to have a higher degree of flexibility. However, thinning the devices causes significant challenges for integrating silicon-based electronics as the interconnect stack for complementary metal-oxide semiconductors (CMOS), for example, is well over 1 μm in thickness and is over 10 μm thick for state-of-the-art CMOS available from foundries. These devices cannot be easily thinned.
Along with the challenges in the lack of flexibility of these silicon-based electronics is the mechanical response associated with embedding them into flexible materials. For example, there is a significant mismatch in mechanical properties of silicon-based electronics (Young's modulus, E≈170 GPa) and soft materials mimicking those of the human body (Young's modulus, E≈100 kPa). This mismatch causes difficulties in the attachment, stretching, and functionality for wearable biomedical instruments. Silicon-based electronics that are rigid and planar have a fracture strain less than 2%, while flexible and stretchable electronics can be bent, stretched, and twisted with typical failure strain greater than 10%.
As another approach to overcome the mismatch problem, thin polymer films that are relatively stiff compared with stretchable materials are embedded into stretchable substrates in order to suppress the onset of interconnect and device breakage. In one example of this approach, patches of polyethylene terephthalate are embedded within a softer polymer to help suppress strain local to the device substrate and increase the shear area, demonstrating operation up to 100% uniaxial stretching and 300% localized internal strain. However, the general intent of locally suppressing strain works when the electronic devices are on the surface of the substrate since no interface exists for normal stress to cause delamination in this configuration.
It would therefore be advantageous to develop a flexible and stretchable substrate incorporating traditional electronic devices that prevents delamination between the materials.